This invention relates to internal reflectance spectroscopic analysis, and in particular to an accessory employing an internal reflection element with a flat sampling surface portion for use with such a technique for the spectroscopic analysis of various samples.
Optical spectroscopy is one of the most powerful and widely employed analytical techniques currently in use. There is a universal demand for a non-destructive, spectroscopic probe of samples of all sizes and in various states, such as powder, solid or liquid. Internal reflectance spectrometry is widely used for this purpose. Since the sample is simply placed in optical contact with a prism, internal reflectance requires little or no sample preparation. In a typical laboratory or factory application, the user purchases an internal reflection accessory which is installed in the sampling compartment of the user's optical spectrometer. As background material, see, for example, "Internal Reflection Spectroscopy", available from Harrick Scientific Corp., and U.S. Pat. No. 4,602,869, the contents of which are herein incorporated by reference. The latter describes an IR accessory, primarily for analysis of liquid samples, having a convenient horizontal sampling surface. However, multiple interactions to enhance the signal strength are not possible in this accessory.
Mirrors (ellipsoids, toroids, parabaloid and spherical mirrors) in attachments or accessories are widely used for IR-VIS-UV spectrometers. These mirrors must be used in an off-axis mode in order to have adequate space to place samples with the accessory between the mirrors to record spectra of the samples. The accessories may be designed for various spectroscopic techniques, e.g., transmission, internal reflection, external reflection, diffuse reflection, etc. Ellipsoids, toroids and parabaloids can be fabricated to produce good imaging for off-axis operation. Spherical mirrors can be fabricated at lower costs and yield good imaging for on-axis or small off-axis angles operation, for example, 5.degree.-10.degree. or less. For large off-axis angles, for example, exceeding 20.degree., serious astigmatism occurs. This astigmatism causes problems in, e.g., re-imaging the light source on the instrument detector element and therefore reduces the sensitivity of the system.
Originally, internal reflectance accessories were designed for use with dispersive spectrometers. These accessories directed the beam to and from the IR elements (IREs). These IREs were typically parallelepipeds or trapezoids and had apertures slightly larger than the slit image of the spectrometer. This produced highly efficient multiple reflection IR accessories. When FT-IR spectrometers began to replace the dispersive instruments, many of the accessories designed for dispersive spectrometers continued to be used. FT-IR spectrometers, however, have beams with round cross-sections and hence do not match the rectangular aperture of the standard IRE. This mismatch results in energy losses that lower the throughput of the IR accessories and reduce the signal-to-noise (S/N) ratio in the resulting spectra.
Two general approaches have been taken to circumvent this dilemma: redesigning the IRE to match its aperture to the FT-IR beam or reshaping the FT-IR beam to match the IRE. In the former category, IREs with square and round apertures have been designed in an attempt to reduce the resulting energy losses. Both configurations have their limitations. The IREs with square cross-sections are designed with apertures slightly larger than the focused FT-IR beam. Like the standard IREs, they have flat sampling surfaces which are easily polished to an optical quality finish and provide scattering-free total internal reflection. These flat sampling surfaces can be oriented horizontally and exposed and are thus ideal for mounting the sample and for applying pressure to obtain optimum contact between the sample and the IRE. However, square cross-sectioned IREs have fewer reflections and hence lower S/N ratios than the traditionally shaped IREs of the same height and length. Use of a longer IRE can increase the S/N ratio, but there is a practical limit to the length based on the width of the sample compartment, the fragility of the IRE, and the ability to achieve good optical contact between the IRE and the sample.
Alternatively, rod-shaped IREs require more expensive transfer optics to direct the entering and exiting beams. These transfer optics are typically difficult to align and result in low throughput. Typically, the throughput is even lower than that of accessories which use rectangular IREs and do nothing to overcome the resulting energy losses. In addition, rods do not produce a well-defined incident angle due to their curved sampling surfaces. These curved surfaces are difficult to polish to a high quality optical finish, resulting in increased scattering losses relative to the standard rectangular IREs. The curved sampling surfaces also limit the types of samples that can be examined. Since solids and powders cannot be clamped onto the crystal, rod IREs are only utilized for liquid sampling.
Internal reflectance beam condensers represent still another approach to solving the FT-IR beam-IRE aperture incompatibility problem. Beam condensers focus and condense the round FT-IR beam into the IRE aperture. This produces a beam that fits within the entrance aperture of the IRE, but it expands within the IRE and completely fills the exit aperture. The beam exiting the IRE has a rectangular cross-section with a width comparable to the diameter of the condensed FT-IR beam and a height several times larger. This elongated beam is directed to the detection optics of the spectrometer. Thus, some fraction of the rectangular beam will not strike the detector. This results in energy losses, low throughput, and a less than optimal S/N ratio.
Harrick Scientific has offered for sale for use in sampling compartments with center focussing optics, a horizontal IRS attachment (Model HRA) for applications requiring an exposed sampling surface. This uses a trapezoidal IRE and plane mirrors to direct the beam to the IRE entrance aperture. But with a round FT-IR beam, light throughput is low and sensitivity suffers.